PHP class that writes session to database

class session_handler {
 protected $maxlifetime = null;
 protected $dbHandle = null;
 public $config = null;
 public static function init($args) {
  return new self($args);
 }
 public function __construct($args) {
  $this->config = $args;
  $this->maxlifetime = get_cfg_var("session.gc_maxlifetime");
  session_set_save_handler(
  array($this, "open"),
  array($this, "close"),
  array($this, "read"),
  array($this, "write"),
  array($this, "destroy"),
  array($this, "gc"));
 }
 public function open() {
  $this->link = mysqli_connect(
  $this->config['host'],
  $this->config['user'],
  $this->config['password'],
  $this->config['database']);
  mysqli_set_charset($this->link, "utf8");
  $sql = 'CREATE TABLE IF NOT EXISTS `%s` (
  `session_id` varchar(255) NOT NULL,
  `session_data` text,
  `session_expires` char(10) NOT NULL,
  PRIMARY KEY (`session_id`)
  ) ENGINE=InnoDB DEFAULT CHARSET=utf8 COLLATE=utf8_unicode_ci;';
  $sql = sprintf($sql, $this->config['table']);
  mysqli_query($this->link, $sql);
  return true;
 }
 public function close() {
  return true;
 }
  public function read($session_id) {
  if(empty($session_id)) return null;
  $sql = 'SELECT `session_data` AS `data` FROM `%s` WHERE `session_id` = "%s" AND `session_expires` > "%u"';
  $sql = sprintf($sql,
  mysqli_real_escape_string($this->link, $this->config['table']),
  mysqli_real_escape_string($this->link, $session_id),
  time());
  $result = mysqli_query($this->link, $sql);
  $row = mysqli_fetch_assoc($result);
  return $row['data'];
 }
 public function write($session_id, $session_data) {
  if(empty($session_id)) return null;
  $newExpires = time() + $this->maxlifetime;
  $sql = 'REPLACE INTO `%s` SET `session_id` = "%s", `session_data` = "%s", `session_expires` = "%u"';
  $sql = sprintf($sql,
  mysqli_real_escape_string($this->link, $this->config['table']),
  mysqli_real_escape_string($this->link, $session_id),
  mysqli_real_escape_string($this->link, $session_data),
  $newExpires);
  $result = mysqli_query($this->link, $sql);
  return mysqli_affected_rows($this->link);
}
 public function destroy($session_id) {
  $sql = 'DELETE FROM `%s` WHERE `session_id` = "%s"';
  $sql = sprintf($sql,
  mysqli_real_escape_string($this->link, $this->config['table']),
  mysqli_real_escape_string($this->link, $session_id));
  $result = mysqli_query($this->link, $sql);
  return mysqli_affected_rows($this->link);
}
 public function gc() {
  $sql = 'DELETE FROM `%s` WHERE `session_expires` < "%u"';
  $sql = sprintf($sql,
  mysqli_real_escape_string($this->link, $this->config['table']),
  time());
  $result = mysqli_query($this->link, $sql);
  return mysqli_affected_rows($this->link);
}
}
class session {
 public static $collection = null;
 public static function open($clean = false, $token = false) {
  if($clean) ob_end_clean();
  if($token) session_id($token);
  session_start();
  self::$collection = $_SESSION;
 }
 public static function id() {
  $num_args = func_num_args();
  if($num_args) {
   $args = func_get_arg(0);
   return session_id($args);
  }else{
   return session_id();
  }
 }
 public static function get($name) {
  return isset($_SESSION[$name]) ? $_SESSION[$name] : null;
 }
 public static function set($name, $value) {
  $_SESSION[$name] = $value;
  return true;
 }
 public static function delete($name) {
  if(!isset($_SESSION[$name])) return null;
  unset($_SESSION[$name]);
  return true;
 }
 public static function destroy() {
  session_destroy();
 }
}
$config = array(
"host"=> "127.0.0.1",
"user"=> "root",
"password"=> "123456",
"database"=> "test",
"charset"=> "utf8",
"table"=> "user_session");
session_handler::init($config);
session::open();
session::set("profile", array("id"=> 1, "user"=> "haowei", "vip-level"=> 6));